There are many applications where it is desirable to recover a metal from solution. For example, precious metals such as gold or silver which are incorporated in a plating or rinse solution may be removed for their intrinsic value.
Another application requiring removal of metals from solution are those where metal pollutants such as mercury, cadmium, copper, nickel, zinc, etc., hereinafter referred to as "heavy metals", must be removed in order to meet or exceed environmental standards. These pollutants typically are contained in waste streams from such industrial sources as metal finishing, metal plating or mining operations. Generally, even a small quantity of a heavy metal can render an otherwise innocuous waste into a hazardous waste, severely complicating disposal. While the removal of these contaminants is desirable from waste solutions, it is usually difficult to achieve separation of just the metals from the solution due to the presence of other waste contained within the stream. For example, the waste stream may include various solid contaminants dispersed therein.
One device for removing heavy metals from waste water is disclosed in U.S. Pat. No. 4,399,020 to Branchick, et al. The device disclosed includes a cell system which accommodates anodes and cathodes for electrolytically removing metals from solutions. Generally, the metal contaminant is plated on the cathode with the cathode then removed and the metal stripped therefrom. The device includes an essentially rectangular tank which includes alternatively solid anodes and cathodes possibly made of a reticulate metallized organic polymer foam. The anodes and cathodes are connectable to oppositely disposed anode and cathode bus bars, respectively. Essentially, a plurality of plates is disposed within the rectangular tank with the metal-containing stream entering at one end and exiting at the other end. An exemplary cell included approximately 41 anodes and was operated in the single-pass mode, relying on inlet filters to remove particulate before the fluid enters the tank. Branchick also requires holes in the reticulate cathodes to prevent overflow should the cathodes become plugged.
While useful in removing metals from solutions which are essentially clean, it has been found that the presence of solids within a waste stream could result in significant buildups of solid material within the tank which eventually reduces substantially the removal efficiency of the electrolytic cells. Utilizing filters prior to entry does not overcome the problem as the filters reduce flow, thereby similarly reducing removal efficiency. Also, some solids still may enter the tank, creating a build-up which eventually requires a system shutdown.
Another problem found with such devices is that utilizing conductive clamping strips wired to an adjacent buss bar, with the strips engaging the reticulate cathode above the liquid level for powering the cathode. Such strips produces an electrolytic gradient across the surface of the cathode producing uneven plating action and reduced efficiency.
Consequently, the search continues for an electrolytic cell capable of removing heavy metals from metal-containing solutions which also include solids dispersed therein.